The instant invention relates to acceleration sensors, or accelerometers, for sensing the acceleration experienced by an object, such as a motor vehicle.
The prior art teaches magnetically-biased accelerometers comprising a housing having an inertial or sensing mass within a cylindrical passage therein which is magnetically biased towards a first end of the passage. Such prior art accelerometers provide a maximum magnetic bias on the sensing mass when the sensing mass is in its "rest" position proximate the first end of the passage. When the housing is subjected to an accelerating force which exceeds this maximum or "threshold" magnetic bias, the sensing mass is displaced within the passage from its rest position therein towards a second position at the other end of the passage. Such displacement of the sensing mass may further be retarded as by using either gas or electromagnetic damping, as taught in U.S. Pat. No. 4,329,549 to Breed, and U.S. Pat. No. 4,827,091 to Behr, respectively. If the acceleration input is of sufficient amplitude and duration, the sensing mass is displaced to the second end of the passage, whereupon the sensing mass operates suitable switch means in the sensor, as by bridging a pair of electrical contacts projecting into the passage at the second end thereof.
Unfortunately, the threshold magnetic bias typically employed by such prior art magnetically-biased accelerometers, i.e., the magnetic bias on the sensing mass thereof when the sensing mass is in its rest position, is equivalent to between about 2 g's to about 8 g's of acceleration. Thus, in the event of a vehicle acceleration (deceleration) of relatively short duration, such as in a 30 mph frontal barrier crash, the high threshold magnetic bias on the sensing mass delays the motion thereof considerably, with the sensing mass ultimately requiring perhaps 20 ms to traverse the length of the passage to operate the switch means of the accelerometer.